A. Field of the Invention.
The present invention is related with the thermal treating of metallic workpieces and more specifically to a method and an apparatus for the strengthening of aluminum workpieces including aluminum wheel rims for vehicles, by means of a continuous thermal treatment.
B. Description of the Previous Art.
Strengthening of metallic workpieces such as aluminum wheel rims, piston head and cylinder assemblies, etc., are conventionally carried out by subjecting said wheel rims to a first thermal treating step at an elevated temperature of about 550.degree. C., then to a quenching step wherein the temperature is drastically reduced at about 150.degree. C. into a cooling fluid chamber, in a less possible time, and then subjecting the cooled wheel rims to a third heating temperature at about 150.degree. C. and diminishing uniformly the temperature to a room temperature.
The conventional process for the thermal treatment of wheel rims comprises placing a batch of 300 to 500 wheel rims on a platform of a furnace wherein said wheel rims are maintained to a temperature of about 563.degree. C. for eight hours; immersing said wheel rims into a cooling chamber by let them fall by gravity into said chamber containing a saline cooling solution, placed under said furnace in order to suddenly cool said wheel rims to a temperature of about 100.degree. C.; then, placing again said wheel rims on the platform to the same furnace in order to reheat again said wheel rims to a temperature of about 150.degree. C. for two hours; and finally recovering said wheel rims by placing the platform at room temperature.
This conventional process has the drawback that it is an energy consuming process. Heat looses are presented each time that the furnace is opened, i.e., firstly on introducing the batch into the furnace, then on immersing said wheel rims into the cooling chamber and finally on opening the furnace to recover said wheel rims, and consequently of the above, great amounts of energy are also necessary in the first and third steps so as to allow said furnace to reach treatment temperatures and maintaining them in equilibrium.
Furthermore, it is a time consuming process because again, each time that the furnace is opened both in the same cycle as well as in each new cycle of treatment, it is necessary to wait a long time to let the furnace to reach the thermal treatment temperatures and the thermal equilibrium.
It is true that the third thermal step, as well as any further steps, such as washing and coating, although necessary, is not a critical one and therefore the energy looses therein could be diminished by carrying it out in a separate stage, however, as to the time and additional equipment is concerned, it is desirable to carrying it out in the same process stage.
Furthermore, in said process when one or more wheel rims of a batch result defective, the entire batch of 300 to 500 wheel rims has to be rejected, with the consequent looses of time, energy, raw materials and money.
Another important drawback of the conventional process is the contamination of the furnace with the oxidant atmosphere and vapors provided by the quenching chamber, causing problems to the process and to the workpieces.
One of the most relevant advances in the art to overcome the problems of the conventional processes and apparatuses is represented by the invention disclosed in U.S. Pat. No. 3,447,788 by providing a quenching chamber separated and independent of a loading chamber and the furnace so as to prevent the oxidant and contaminant vapors to enter into the furnace and wherein the workpieces are firstly charged on a loading chamber which afterwards is shifted to a sealed closed heating chamber, then shifted back to the loading chamber and finally transferred to the quenching chamber.
Although said process and apparatus avoid problems of contamination of the heating chamber, still have problems of energy and time consumption because the heating steps are carried out in the same heating chamber which have to reach their equilibrium temperature in each step and are not continuous. There are continuous processes for the annealing and coating of metallic workpieces, but such processes are limited only to continuous and flexible metal strips or wire, as disclosed in U.S. Pat. Nos. 3,511,686 and 4,295,033.
All the above disclosed drawbacks of the processes and apparatuses of the previous art are overcomed by the continuous process and apparatus for the thermal treatment of metallic workpieces, of the present invention, comprising placing a batch of two or four workpieces on carrier means such as a basket hinged on a continuously advancing conveyor means such as a conveyor chain; introducing said workpieces in said basket through a first instantaneously opening and closing gate into a uniformly heated first thermal treating step wherein said continuously advancing workpieces are heated at a temperature of from about 500.degree. C. to about 600.degree. C. and preferably 560.degree. C.; introducing the heated workpieces in said basket through a second instantaneously opening and closing gate into a second thermal treatment step placed immediately after the end of the first step and in communication therewith, by lifting a vat containing a cooling fluid to immerse said workpieces in said basket into the cooled fluid of said vat in order to rapidly cooling said workpieces to a temperature of from about 50.degree. C. to about 150.degree. C. and preferably of about 100.degree. C. and then lowering said vat; and optionally introducing said cooled workpieces in said basket through a third instantaneously opening and closing gate into a third thermal treatment step placed immediately after the end of the second step and in communication therewith, wherein said continuously advancing workpieces in said basket are heated to a temperature of from 150.degree. C. to about 250.degree. C. and preferably of about 200.degree. C.
In this way, by having the two or three thermal treating steps isolated from each other but placed in communication one immediately after the other, and by providing a cooling vat which is easily lifted and lowered so as to allow the hingeable baskets with the metallic workpieces to be immersed into said vat, and by providing the instantaneously opening and closing gates between each step, contamination with the oxidizing vapors of the cooling chamber, to the thermal treatment steps is avoided.
Furthermore, by providing the thermal treatment steps placed one immediately after the other and by having them isolated from each other by means of their respective instantaneously opening and closing gates, the thermal treating time is reduced from eight hours in the first step of the conventional process and apparatus, to only five hours in accordance with the present invention, with the consequent reduction in the overall time.
Obviously, it has to be understood that the critical steps of the thermal treatment comprise only the heating and quenching steps and that the third thermal step can be optionally carried out in a separate stage, however, as already disclosed, it is a matter of convenience that this third step, as well as washing and coating steps, be carried out in the way previously disclosed to optimize energy, time and money savings.
Additionally, by continuously providing batches of at least two or four workpieces to the thermal process and apparatus, rejection of large batches of workpieces is avoided.